Just as the name suggests, this feature would allow for players to be able to go into their TARDIS settings, and change/adjust the de/re-mat sound effects.

How is this feature accessed/used?  This can be done via an option in the TARDIS settings that would open an interface containing one or more slidebar(s) that allow for adjustment to the pitch, bass, and any other factors that may apply. Presets can even be added to assist in providing a jumpstart to finding the right levels for the player. There would also be a reset option to change the sound effect back to the default TARDIS demat and remat.

Is this feature survival friendly? For all intents and purposes, yes and no. It would not be a feature that provides an advantage to the player in a survival environment, it would be a customization feature only. On a multiplayer standpoint, however, it may be possible for a player to be able to recognize whose TARDIS was landing in the distance, just be hearing it; assuming they memorized the sound effect their friend or foe made for their TARDIS. 

Is this feature creative friendly? Very. It's mainly to give more customization freedom to the player. With creative mode, building a custom TARDIS interior is now a piece of cake, and with this feature, they can add a custom sound effect to go along with it. They would go together like bread and butter as it will.

How would this feature benefit the mod as a whole?  Remember when I mentioned presets? Throughout Doctor Who's history, with each new TARDIS, especially TARDISes that didn't belong to The Doctor, the sound effect was always a bit different. This feature would make each of the TARDISes in a mod more accurate to the show and the audio dramas. A handful of interiors would, of course, retain the default sound effect(s), but for example, ones like the 1996 Movie interior would have that more shrill yet-bassed sound effect mainly heard in the audio dramas,  there could even be a preset for a Type-45 TARDIS, The TARDIS model the Master was known to use for a while, sound effect; where it always had that strange treble effect in the background. There could even one for The Rani's interior, where it had more of an echo and higher pitch to it. These are just examples of the presets that could be added to this feature. 

In a nutshell, this feature would provide more accuracy and customization to the TARDISes as a whole. Have a great day, :)

I like this idea, except there is only one demat/remat sound In doctor who

In the TV series perhaps, but there have been several in the Big Finish audio dramas, and a few in the classic series. 

O idk